Applicant's previous inventions in this field are disclosed, for example, in published UK Patent Applications GB 2 341 738 A and GB 2 341 737 A, to which the reader is referred for necessary background to the present invention.
GB 2 341 738 A (“Invention A”) relates to power line protection using known distance protection techniques but modified according to Invention A to operate in two different modes, namely instant and delayed operation modes. Using suitable algorithms, Invention A enables power line protection apparatus (so-called “relays”) to make the decision whether to operate the associated circuit breaker(s) instantly or after a delay, depending on the system and fault conditions.
On the other hand, GB 2 341 737 A (“Invention B”) relates to power line protection using known directional overcurrent techniques but modified according to Invention B to operate in an accelerated mode. Using suitable algorithms, Invention B enables relays to make an accelerated operation decision depending on system and fault conditions, therefore significantly reduce the operating time in places where known directional overcurrent relays cannot offer satisfactory performance. To achieve a successful identification of the power line section containing the fault, the algorithms take into account the level of load current in each phase.
The present invention further improves these two previous inventions in that it deals with problems presented by power lines and cables having in-zone tapped-off loads.